Selectivity of Ion Exchange

The selectivity of ion exchange interactions depends on the relative binding strengths of ions to a site, where the site may be a molecule free in the cytoplasm, a carrier molecule in a membrane, a surface of a membrane or a precipitated phase, or part of a channel or a pump for moving ions across membranes. The affinity for the site can usually be expressed in a very simple form as a binding stability constant, K, for the equilibrium:

The concentration of X is here treated in mass action equations and takes a similar form for surface sites or sites in free solution. In the case of surface sites, the equation takes the form of a Langmuir isotherm.

Selectivity of binding depends on the size and charge of an ion in the first instance. It would be expected that small highly charged ions of opposite sign would bind together best, but this expectation is not fully borne out in practice, since competition for a site also depends on constraints due to hydration of the ions:

Since small, highly charged ions are the most strongly hydrated, making for competition between H2O and

Figure 6 The molecule shown at (A) can bind the monovalent ions selectively because of how it wraps around them (C). The hole in the middle selects the size of the ion, as shown by the binding constants (B). The best binding is for potassium. Such molecules can pick up cations and transfer them across membranes, exchanging the ion for other ions according to binding strengths and concentrations. Many antibiotics are based on such exchange possibilities.

Figure 6 The molecule shown at (A) can bind the monovalent ions selectively because of how it wraps around them (C). The hole in the middle selects the size of the ion, as shown by the binding constants (B). The best binding is for potassium. Such molecules can pick up cations and transfer them across membranes, exchanging the ion for other ions according to binding strengths and concentrations. Many antibiotics are based on such exchange possibilities.

X, there is the possibility of matching ions, M, of different charges and sizes with particular kinds of designed exchange site, X. Controlling factors now for cations, M, are the charge on X and the steric restrictions present in X or induced in MX on binding. Real examples illustrate the point that the binding of cations M to sites X can be in almost any order. The case of the preferred selection of K+ over the smaller and larger ions Li+, Na +, Rb + and Cs+ illustrates the point in Figure 6. In all cells the K + channel virtually excludes Na +. Similarly, the Ca2 + pump of most cells excludes Mg2 +. In both bases the larger ion is preferred due to the size of the cavity and the accepting anion, X.

For trace element metal ions such as those of iron (Fe2 + , Fe3 + ), zinc (Zn2 + ) or copper (Cu + , Cu2 + ), facts other than charge and size influence selectivity. They are the ability to form covalent bonds depending on the electron affinity of the cation and stereochemical preferences of the metal ions due to their polarizability. Again, examples illustrate these points.

A very important distinction between the bindings of the major metal ions, Na +, K +, Mg2 + and Ca2 + and the trace elements, is that the binding units X for the former generally employ organic molecules containing oxygen (O) donor centres only, while for the trace elements the group X may utilize nitrogen (N) or sulfur (S) donors. Examples are given in Table 5, Amongst the trace elements the strength of binding follows the general series of divalent M2 + ions:

The additional selectivity factor arises due to the stereochemical preferences of these ions.

We can now consider the cytoplasm of a cell as a solution of ion exchange centres, often proteins, which can all bind M but with different affinities.

Together with the fact that the amounts of ions, M, in the cytoplasm varies from 10mol L"1 (Na +, K + ) to 10~17 molL"1 (Fe3 +), this chemical selectivity leads to a virtually complete fixation of the M distribution on different X centres. However, it must not be forgotten that these associations are not permanent and exchange takes place all the time. Many sites will only be occupied preferentially, not specifically, by a given cation. This is particularly important when the environment becomes polluted.

The selective uptake of anions follows similar properties based on size, charge and steric constraints. However, covalent attachment is much less significant than hydrogen bonding. The ability to form hydrogen bonds by anions appears to be related to surface charge density. Thus, F_ and OH~ readily form H bonds when compared with Cl_,Br_ or SH. Once again, the anions exchange quite rapidly with organic surfaces.

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